1. Field of the Disclosure
The present invention relates to a device and a method for measuring a cornea. The present invention relates, in particular, to an ophthalmological device and an ophthalmological method in the case of which, for the purpose of measuring the cornea, a two-dimensional reference pattern is projected onto the cornea and a reflection pattern reflected by the cornea by virtue of the reference pattern is acquired.
2. Related Art
Keratometers (or ophthalmometers) are used to measure the surface curvature of the cornea and determine the corneal profiles. In this case, the reflection of an illuminated reference object on the cornea is acquired, and on this basis the curvature of the reflecting corneal surface is determined. In known videokeratometers, a number of concentric rings are projected onto the cornea. In this procedure, it is preferred to respectively provide or illuminate a diffusely scattering body (screen) with a pattern (for example placido pattern) which is reflected by the eye and recorded by a camera located mostly on the visual axis of the eye. In addition to radial ring patterns, there are also other discrete patterns with binary light/dark structures such as, for example, two-dimensional chessboard patterns. Proceeding from the deformation of the rings on the cornea together with additional assumptions relating to the distance of the eye, that is to say relating to the location of the reflection on the cornea, a determination is made of the surface inclination of the cornea, which enables conclusions to be drawn on the refractive power of the cornea. On the basis of an expected geometrical configuration (shape) of the eye, it is also possible to determine a three-dimensional form of the cornea by integrating the radial surface inclination. By virtue of the low number of the radial interpolation points, that is to say the low number of rings or light/dark transitions (typically between fifteen and twenty rings), these measurement methods or measuring instruments are not very accurate and are affected by errors because of their limited radial resolution.
In order to improve the accuracy and the measurement range, U.S. Pat. No. 5,953,100 proposes the use of a plurality of cameras which detect reflections at the eye from various perspectives. However, because of the specular reflection of the eye a corresponding object point of the eye cannot be made out in the views of the camera (that is to say a triangulation is impossible). Consequently, each image of a camera view must be evaluated per se, and the data must be merged in a concluding step. Such methods raise the accuracy only conditionally. The number of the measuring points rises maximally in proportion to the number of cameras used.
In order to determine the corneal topography in accordance with the above method, patent specification U.S. Pat. No. 6,926,408 proposes a continuous two-dimensional pattern which uses a sinusoidal radial intensity profile and a sinusoidal angle-dependent colour profile. The pattern is generated either as a flat shape and fitted on a translucent conical shape, or the colours are applied directly to a suitable surface. The patient is positioned with his eye in front of the two-dimensional pattern, and a CCD (Charged Coupled Device) camera is used to detect the reflection pattern reflected on the cornea. In order to determine the corneal topography, the pixels of the acquired reflection pattern are respectively correlated with the corresponding reflection points on the cornea. Using a continuous reference pattern increases the number of potential measuring points by comparison with the discrete patterns mentioned at the beginning, and this allows an improvement in measuring accuracy. For a more robust image processing, U.S. Pat. No. 6,926,408 proposes the application of bandpass filters, but this reduces the resolution since, depending on the filter (in particular, on its centre frequency and bandwidth), it is respectively only filtered-out parts of the acquired reflection pattern which are evaluated, and these have a lower resolution than the unfiltered reflection pattern. Bandpass filters function better the more continuous the signal and the smaller the frequency modulation. These conditions do not obtain on the eye because of the patient-dependent variability, that is to say the strong distortion of the ring pattern, and interrupted rings (for example by eyelashes). In other words, although the use of a continuous reference pattern increases the resolution by comparison with discrete patterns, the application of bandpass filters to the corresponding reflection pattern does not lead to a continuous measurement which measures each pixel per se. Because of the slight advantages of the abovenamed practical limitations, methods with continuous patterns have not yet reached product maturity. Methods which measure each pixel per se and thus attain a maximum local resolution or number of measuring points, which lies an order of magnitude above previous methods, do not exist at present.